> Svelte gives you an elegant way to use CSS in your components with <style> tags, why not implement transitions and animations in CSS there?

Because CSS has no hooks into HTML lifecycle. If you want to nimate something that appears in the DOM, or disappears from the DOM elegantly, CSS ain't it.

That's why almost every single framework outside Svelte struggles with animations and employs increasingly bizarre and brittle workarounds to make them work.